MicroRNAs are short RNA sequences that turn genes on or off. To see which are involved in heart-cell division, Giacca's team tested 875 human microRNAs in cultured rodent heart muscle. They found 204 that reactivated cell proliferation, including two that affected nearly 2000 genes.

The team induced heart attacks in adult mice and then injected the edge of the damaged area with a harmless virus carrying the two most potent microRNAs.

Two weeks later, the extent of the damage was much less in mice that were injected with the microRNAs, and the strength of contraction and other measures of heart performance were better preserved. What's more, the affected tissue was full of mature muscle cells that were multiplying.

Giacca says this could be a valuable therapeutic intervention after heart attacks in humans, as long as the microRNAs don't cause other kinds of cells to divide, possibly causing cancers (Nature, DOI: 10.1038/nature11739).

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